Such vegetable oils as soybean oil, rapeseed oil, and castor oil are among conventional electric insulating oils applied to transformers, cables, circuit breakers, and capacitors for their insulation and cooling.
They have recently been replaced by mineral insulating oils or synthetic insulating oils. The former is produced from heavy crude oil by vacuum fractional distillation and subsequent purification (such as washing with sulfuric acid, alkali, and water, and clay treatment). The latter is prepared from diphenyl, silicone, phthalate ester, and the like.
Mineral insulating oils, however, are likely to be restricted in their use because of their high flammability (which endangers safety) and their possibility of posing problems with energy and environment.
On the other hand, synthetic insulating oils have the disadvantage of being highly flammable and expensive. Moreover, phthalate esters are said to cause endocrine disruption.
PCB, which was used as electric insulating oil for a certain period in the past, has been banned because of its problems with safety, toxicity, and environmental pollution.
The foregoing has turned public attention to switching conventional electric insulating oils to safe natural vegetable oils such as soybean oil, rapeseed oil, and castor oil. However, vegetable oils are not suitable for large transformers (which are cooled by convection of insulating oil) on account of their high viscosity and high pour point. Therefore, it has been common practice to use vegetable oils (as electric insulating oil) in combination with mineral or synthetic electric insulating oils.
Mixing vegetable insulating oil with mineral or synthetic insulating oil does not solve problems inherent in the latter.
There has recently been proposed an electric insulting oil derived from a lower alcohol ester of vegetable oil such as rapeseed oil, corn oil, safflower oil, and the like. (See Japanese Patent Laid-open Nos. Hei 9-259638, Hei 11-306864, and 2000-90740.)
These insulating oils, however, are not suitable to practical use because of their incompletely reduced viscosity and pour point and their poor stability to oxygen and heat, and they need improvement.
Rapeseed oil, corn oil, and safflower oil listed as vegetable oils in the above-mentioned documents are not necessarily regarded as renewable resources from the standpoint of the amounts and districts of worldwide production. It is desirable to select insulating oils from a broader range of vegetable oils.